The present invention relates to a resin composition, and more particularly to a resin composition comprising a polycarbonate resin and a graft-copolymer prepared by graft-polymerizing a monomer mixture onto a rubber polymer, which is excellent in thermal stability, weatherability and impact strength at a low temperature and suitable for use of outside parts for automobiles.
Recently, engineering plastics such as polycarbonate resins have been extensibly used for outside parts for automobiles. They have been required to have the excellent impact strength at a low temperature in a temperature range of -40.degree. to -10.degree. C., in addition to the excellent thermal stability on on-line coating and weatherability. The impact strength at low temperature is shown by a transition temperature from a ductile fracture region to a brittle fracture region (a brittle-ductile transition temperature, hereinafter referred to as "B-D transition temperature"). The lower the B-D transition temperature, the more excellent the impact strength at low temperature.
Although the polycarbonate resin is an engineering plastic having excellent impact strength, weatherability, machanical properties and physical properties, its B-D transition temperature is high compared to the above-mentioned temperature range. Accordingly, the outside parts for automobiles prepared from the polycarbonate resin alone is insufficient in impact strength at low temperature.
Nevertheless, methods for improving the B-D transition temperature of the polycarbonate resins have hitherto been scarcely proposed, and some methods for improving the impact strength at room temperature or the processability, as mentioned below, have been merely proposed.
For instance, Japanese Examined Patent Publication No. 38-15225 proposes a method wherein an ABS resin prepared by graft-polymerizing styrene and acrylonitrile onto a butadiene rubber polymer is admixed with a polycarbonate resin. Also, Japanese Examined Patent Publication No. 39-71 proposes a method wherein an MBS resin prepared by graft-polymerizing methyl methacrylate and styrene onto a butadiene rubber polymer, which has hitherto been known as a reinforcement for vinyl chloride resins, is admixed with the polycarbonate resin.
The resin compositions according to the above-mentioned Japanese publications, however, have defects that the thermal stability and weatherability are poor, caused by, probably, the butadiene components.
On the other hand, Japanese Examined Patent Publication No. 48-29308 proposes that in order to improve the thermal stability and weatherability, a graft-copolymer using an alkyl acrylate polymer as a rubber component is admixed with the polycarbonate resin.
According to the above-mentioned method, as the alkyl acrylate, alkyl acrylates having an alkyl group with 5 or less carbon atoms, particularly n-butyl acrylate, are mainly used because they are cheap and can be easily handled. Although the above-mentioned composition can be improved in thermal stability and weatherability, it is higher in B-D transition temperature than the composition using the butadiene rubber polymer.
As mentioned above, a resin composition containing the graft-copolymer which is satisfactory in all of the thermal stability, the weatherability and the impact strength at low temperature showing by the B-D transition temperature cannot be obtained according to the conventional methods.
An object of the present invention is to provide a resin composition comprising mainly a polycarbonate resin which is excellent in all of the thermal stability, the weatherability and the impact strength at low temperature.
This and the other objects of the present invention will become apparent from the following description hereinafter.